The invention relates to a method and a device for changing the spacing at which at least one tool, which is disposed on the radial outside of an associated rotating roller, engages with a web of material running past.
Tool-equipped rollers are used to process material webs running past them, for example as cross cutters for paper. A paper cross cutter is used to cut a paper web—which is wound on a roll—by means of a tool in the form of a knife, into individual segments that usually correspond to standardized formats. The current invention can be used in all machines in which a chronologically periodic movement of a tool occurs, for example crosscut saws or bag forming, filling, and sealing machines, and the like.
U.S. Pat. No. 5,662,018 has disclosed a paper cross cutter that is comprised of two opposing rollers, which are each equipped with a number of knives disposed on the radial outside, in order to cut a material web—which is being conveyed between the two rollers—into predetermined segments. During the engagement of the tools with the material web, the circumferential speed of the tool is synchronized with the speed of the material web in order to permit the cut to be executed along a correct line. This device also makes it possible to cut the material web in different formats. A changeover of the consequently required engagement spacing of the knife-like tool is achieved by virtue of the fact that different tools are associated with different engagement spacings, where of the tools that are kept on hand, the tools that produce the desired engagement spacing are the ones that are brought into engagement. The tool engagement is controlled mechanically by means of hydraulic adjusting mechanisms. A disadvantage of this changing of the engagement spacing lies in the rather complex design of the device, which is expensive to assemble and is subject to wear due to the large number of individual mechanical parts.
By contrast, DE 36 08 111 C1 has disclosed changing the engagement spacing of a tool through the use of electronic means. Two opposing rollers, each of which is provided with a respective tool in the form of a knife, are driven directly by means of respectively associated electric motors. The upper roller is driven with the lower roller by means of a gear transmission in order to synchronize the rotating rollers.
Special d.c. motors are used as the electric motors, which are driven discontinuously by a shared electronic control unit, as a function of various input data, i.e. in a manner similar to disk-cam control. The input data for the control unit include the circumferential speed of the tool detected by an incremental transducer as well as data regarding the speed of the material web traveling through the rollers. It is also possible to preset a desired engagement spacing of the tool, i.e. a desired format length. Depending on these data, the two special d.c. motors are controlled so that starting from a standstill, they can be brought extremely quickly to their full speed, which corresponds to the respective speed of the material web. This speed is kept constant for the entire duration of the cutting procedure. Depending on the desired format length, the motor can be adjusted down to a lower speed and then for the cutting procedure, can be accelerated back to the full speed. For smaller formats, after passing the cutting speed, the motor can be accelerated to higher speeds in order to rotate the roller at an increased speed and to execute the cut before the material web has traveled the distance of the so-called synchronous format, which corresponds to the revolution circumference of the knife.
In the above-described known method for determining the spacing at which knives, which are disposed on the radial outside of rotating rollers, engage with a passing material web, the problem arises that although it is possible to change the engagement spacing during operation, i.e. to change the format length, waste is produced during the time it takes to change the format length. In order to avoid generating such waste while changing the engagement spacing, it was previously customary to first turn off the machine, set the machine to the new engagement spacing, and then turn the machine back on. On the other hand, however, this procedure requires a lot of time, which is disadvantageous in mass production, and generates waste during the run-up and braking.